A large eddy simulation (LES) study of the flow around a 1/4 scale squareback Ahmed body at $R{e}_H=33,333$ is presented. The study consists of both wall-resolved (WRLES) and wall-modelled (WMLES) simulations, and investigates the bimodal switching of the wake between different horizontal positions. Within a non-dimensional time-window of 1050 convective flow units, both WRLES and WMLES simulations, for which only the near-wall region of the turbulent boundary layer is treated in a Reynolds-averaged sense, are able to capture horizontal (spanwise) shifts in the wake’s cross-stream orientation. Equilibrium wall-models in the form of Spalding’s law and the log-law of the wall are successfully used. Once these wall-models are, however, applied to a very coarse near-wall WMLES mesh, in which a portion of the turbulent boundary layer’s outer region dynamics is treated in a Reynolds-averaged manner as well, large-scale horizontal shifts in the wake’s orientation are no longer detected. This suggests larger-scale flow structures found within the turbulent boundary layer’s outer domain are responsible for generating the critical amount of flow intermittency needed to trigger a bimodal switching event. By looking at mean flow structures, instantaneous flow features and their associated turbulent kinetic energy (TKE) production, it becomes clear that the front separation bubbles just aft of the Ahmed body nose generate high levels of TKE through the shedding of large hairpin vortices. Only in the reference WRLES and (relatively) fine near-wall mesh WMLES simulations are these features present, exemplifying their importance in triggering a bimodal event. This motivates studies on the suppression of wake bimodality by acting upon the front separation bubbles.

一个大涡模拟（LES）研究1/4比例方背Ahmed体周围的流动 $\mathrm{[R}{Ë}_H=33，333$被表达。这项研究包括墙解析（WRLES）和墙建模（WMLES）模拟，并研究了尾迹在不同水平位置之间的双峰切换。在1050个对流流动单元的无量纲时间窗口内，WRLES和WMLES模拟（仅湍流边界层的近壁区域在雷诺平均意义上得到处理）都能够捕获水平（跨度）改变尾流的横流方向。成功地使用了Spalding定律和墙的对数定律形式的平衡墙模型。但是，一旦将这些壁模型应用于非常粗糙的近壁WMLES网格，其中也会以雷诺平均的方式处理湍流边界层外部区域动力学的一部分，不再检测到尾流方向的大规模水平移动。这表明在湍流边界层的外域中发现的大规模流动结构负责产生触发双峰切换事件所需的临界流量间歇性。通过查看平均流动结构，瞬时流动特征及其相关的湍动能（TKE）产生，可以清楚地看到，在艾哈迈德身体鼻子后部的前分离气泡通过散发大的发夹状涡流而产生高水平的TKE。这些功能仅在参考WRLES和（相对）精细的近壁网格WMLES仿真中存在，从而说明了它们在触发双峰事件中的重要性。